The present invention addresses the problem of creating an effective actuator to provide tactile feedback to an individual in a high workload environment. Such an environment is represented by flight conditions which could cause spatial disorientation to a pilot, underwater navigation for a SEAL diver, or EVA missions for a space walker. In each situation, the opportunity to communicate by using the sense of feel opens an underutilized message path in addition to the conventional visual and aural techniques. The present invention relates generally to apparatus for providing tactile sensation to a user.
The complexity of the military environment places great physical and mental stress on soldiers involved in performing complex tasks. For example, aircraft pilots report that flying in conditions of poor visibility or at night can lead to spatial disorientation which may, in turn, lead to the necessity for intervention to preserve the aircraft (NAVATHE and SINGHE, 1994). RUPERT et al. (1989) and CLARK and RUPERT (1992) reviewed surveys on aircraft accidents and determined that nearly 4 to 10% of Class A mishaps ($500,000 damage or loss of life) and 10 to 20% of the fatal aircraft mishaps were a result of inadequate spatial awareness. Similar situations are found in underwater diving under conditions of poor visibility or in battlefield situations requiring attention to a large battery of sensor information.
Aircraft safety is a major concern for both the public and private sectors of the aeronautical industry. Audio contact, even if initially available, is often lost in emergency situations. Conditions which arise during emergencies, such as dense smoke, render visual cues useless. Needs exist for apparatus that allow pilots and other aircraft personnel to receive information and instructions when visual and audible aids are not available.
In environments where audio and visual information channels would only compete with data necessary for optimal performance of complex military tasks, tactile sensations can be valuable in providing an additional information channel. Tactile sensations can be used to improve the situational awareness of military personnel and to guide military personnel in the performance of tasks requiring input from external sensors and control systems. Work performed at the Naval Aerospace Medical Research Laboratory (NAMRL), National Aeronautics and Space Administration, and University of West Florida has demonstrated that orientation awareness can be maintained through tactile cues (RUPERT et al., 1994). Successful development of tactile interfaces to improve situational awareness requires the availability of actuators capable of providing tactile stimuli representative of the operational situation. These actuators have been termed "tactors".
A wide range of actuators have been suggested for use as tactors in improving situational awareness. These include actuators based on piezoceramic structures, linear motors, shape memory alloy materials, magnetic materials, magnetostrictive materials and variable reluctance devices. The specifications for these tactors are as variable as the operational environments for expected use but include capability to generate tactile signals of sufficient magnitude; small size; low energy consumption; low hazard of electrical shock; quiet operation for use in undersea operations; controllable signal duration, intensity and frequency to allow use by individuals having a range of tactile and discomfort thresholds.
The impetus for the use of tactile stimulation to convey information about the environment may be traced to efforts at sensory substitution in providing a tactile channel for individuals with visual or auditory impairment (BLISS et al., 1970, WEISENBERGER and RUSSELL, 1989). More recently, tactile devices have been of great interest in the development of virtual reality displays for telerobotics, remote control and simulation (DE ROSSI, 1991, BROOKS et al., 1990). The physiological basis of tactile stimulation in producing a haptic display has been reviewed by CHOLEWIAK and COLLINS (1991). Tactile sensations may be perceived by free (bare) nerve endings or by nerve endings associated with or encapsulated within accessory structures. The current state of knowledge about these receptor structures is somewhat inconclusive in identifying the specific interactions responsible for the perception of tactile stimuli, but considerable effort has been expended to understand the action and characteristics of receptors in haptic perception. The encapsulated tactile receptors include the Pacinian corpuscles, Merkel's disks, Ruffini cylinders and Meissner's corpuscles. Each of the encapsulated tactile receptors is different from the others from an anatomical standpoint with resulting differences in response.
The Pacinian corpuscles respond to vibrations with displacements as small as 0.2 .mu.m at a frequency of 250 Hz. The threshold increases for vibratory stimuli outside of the optimal frequency range. Increase in the size of the contactor decreases the threshold through an effect known as spatial summation. Non-Pacinian receptors have not been thought to exhibit spatial summation. The characteristics of probable tactile receptors are given in Table I.
The use of vibrotactile displays for sensory substitution systems has been reviewed by KACZMAREK et al.,(l 99 1) who presented a discussion of the information capacity of the tactile channel. The tactile information system may be said to possess some 10,000 parallel channels (receptors) capable of responding to stimuli as short as 10 ms, thereby providing considerable informational capacity if properly addressed. The information bandwidth of the tactile channel may vary widely among individuals. It is well known that vibrotactile stimulation thresholds vary considerably among individuals and with age. Considerable research expended toward determining the vibrotactile threshold has been reviewed by MAEDA and GRIFFIN (1994). Additional considerations in the application of tactile stimuli to improve situational awareness may be drawn from research by POST et al., (1994) who showed that the subjective intensity of suprathreshold tactile stimuli is considerably reduced during motor activity. For this reason, it is expected that higher amplitude tactile signals will be necessary to convey information to individuals engaged in active motor tasks.
The use of piezoelectric materials in actuators is reviewed by DAMJANOVIC and NEWNHAM (1992) and by NEWNHAM and RUSCHAU (1991). Typically piezo-materials displace less than 1 .mu.m under an applied electric field. To increase the displacements, several designs have been introduced in the literature such as stacks, unimorph, bimorph, and Rainbow.TM. (ELISSALDE and CROSS, 1995). Piezoelectric multilayer stacks can be fabricated by joining multiple piezoelectric rings or plates, such that the total displacement of the stack is the sum of the displacements of each individual plate. Adjacent plates are separated by inner electrodes to provide vertical displacement through the piezoelectric charge coefficients. Several hundred plates are necessary to provide total displacements on the order of 20 .mu.m (LUBITZ and HELLEBRAND, 1991).
The standard unimorph is made up of a flat piezoelectric wafer bonded to a metallic shim from one side. To increase the displacement range, the bimorph structures utilize PZT layers attached to both surfaces. The unimorph and bimorph structures usually utilize the geometry of a cantilever beam. In the bimorph, the application of an electric field across the two outer layers causes one layer to expand while the other contracts. This results in a bending motion with relatively wide displacements at the tip of the cantilever beam. In a cantilever configuration, the displacement of the tip, y, is related to the length of the cantilever, L, the applied voltage, V, and the thickness of the cantilever, h: EQU y=2L.sup.2 Vd.sub.31 /h.sup.2,
where d.sub.31 is the piezoelectric charge coefficient showing the magnitude of the strain induced in the direction of the cantilever when electrodes are applied perpendicular to the long axis of the cantilever. The magnitude of d.sub.31 is typically less than 300.times.10.sup.-12 V/m Cantilever-based piezoelectric actuators require lengths on the order of 25 mm or more to achieve a free deflection of 0.3 mm. Further increase in the displacement range may require utilizing resonance regimes.
Structurally similar to piezoelectric unimorphs, Rainbow.TM. high displacement domed buckling actuators produce displacements approximately 10 times greater than bimorphs ("Medical Equipment and Design," 1994). They support moderate loads up to 9 kg while achieving large displacements of between 0.025 and l.27 mm, however, with a relatively high voltage range of 100-500 volts.
At present, no single tactile actuator has shown the capability of meeting each of these requirements. To address this problem, American Research Corporation of Virginia (ARCOVA) has developed piezoceramic vibrotactile transducers capable of conveying tactile information to the user through layers of clothing. The overall goal of was to design and develop new vibrotactile transducers capable of safely producing adequate skin and surrounding tissue stimulus at low energy consumption.